Both major compilers yell at you for this nowadays... it's pretty unforgivable IMHO for somebody to be asking it as an exam or interview question if the right answer isn't "undefined":

    <source>:5:10: warning: multiple unsequenced modifications to 'a' [-Wunsequenced]
        5 |     a = a++ + ++a;
          |         


    <source>:5:7: warning: operation on 'a' may be undefined [-Wsequence-point]
        5 |     a = a++ + ++a;
          |     ~~^~~~~~~~~~~